The switch was made because the Roman numeral for 50 is "L." ESPN.com's Darren Rovell received an explanation from Jamie Weston, the league's vice president of brand and creative:

When we developed the Super Bowl XL logo, that was the first time we looked at the letter 'L.' Up until that point, we had only worked with X's, V's and I's. And, at that moment, that's when we started to wonder what will happen when we get to 50?